Electron image intensifying devices



Feb. 1, 1966 J. D. M GEE ELECTRON IMAGE INTENSIFYING DEVICES Filed Sept. 18, 1962 United States Patent 3,232,781 ELECTRON IMAGE INTENSTFYIN G DEVICES James Dwyer McGee, London, England, assignor to National Research Development Corporation, London, England, a British corporation Filed Sept. 18, 1962, Ser. No. 224,494 Claims priority, application Great Britain, Feb. 3, 1958, 3,453/58 4 Claims. (Cl. 117-335) F This application is a continuation-in-part of my application Serial No. 789,926, filed January 29, 1959, and now abandoned.

This invention relates to multiple layer electron image intensifying screens, to methods of preparing such screens and to electron optical devices using such screens. One type of such electron optical device comprises means for creating an electron image, the electrons of which fall upon a fluorescent layer in which the electrons liberate photons of light. This light then falls upon a photocathode from which electrons are liberated which are greater in number than the electrons incident upon the fluorescent layer.

Activated zinc sulphide has long been used in a crystalline podwer form as a phosphor. More recently it has been known that a transparent amorphous layer of zinc sulphide can be formed which layer can then be activated to give a fluorescent efliciency comparable with that of the powder phosphor. A technique for the evaporation of zinc sulphide to form a transparent amorphous layer on a substrate such as glass and the subsequent activation of the Zinc sulphide layer by baking at a temperature of about 1,000" centigrade, has been described by C. Feldman and M. OHara in the Journal of the Optical Society of America, 47, No. 4, .April 1957.

Other phosphors such as willemite (ZZnOSiO and calcium tungstate may also be used in this invention, and may be deposited by similar techniques.

The main objects of the present invention are to provide a thin multiple layer electron image intensifying screen having a total thickness not exceeding 20 having good image definition and comprising a light-transmitting layer of phosphor, for example activated zinc sulphide, and a photo-electric layer, for example antimony activated with cesium, and to provide such a screen in which there is no separating layer between the phosphor layer and the photo-electric layer.

A multiple layer electron image intensifying screen according to the present invention comprises a lighttransmitting layer of fluorescent material in the amorphous state having a thickness between 1 and an electron permeable supporting carrier layer adjacent one side of the. fluorescent layer and having a thickness not exceeding in, a photo-electric layer adjacent the other side of the fluorescent layer, and an electron permeable metallic layer adjacent the carrier layer, the total thickness of the screen being not more than g.

One method according to the present invention of preparing the electron image intensifying screen comprises the steps of forming the light-transmitting layer of fluorescent material on the supporting carrier layer, reactivating the layer, if necessary, to form a fluorescent layer, coating the fluorescent layer on the exposed face with the photo-electric layer, and coating the exposed face of the carrier layer with the electron permeable metallic layer.

The fluorescent layer may be of zinc sulphide, willemite, calcium tungstate or other known fluorescent material.

The carrier layer on which the potentially fluorescent layer is deposited must be thin if good image definition is to be obtained and may be of aluminum oxide, magnesium oxide, wire mesh or other suitable inert heat resistant material. Aluminum oxide is particularly suitable for use as a carrier since thin films of about 0.1 1. thick may be formed having desirable heat resistance and strength. Such carrier film need never therefore have a thickness greater than, say 1 w.

The photo-electric layer may be formed. by depositing a layer of antimony and subsequently activating the antimony with cesium vapor to form a photo-electric layer.

The antimony layer protects the activated Zinc sulphide layer from attack by the cesium vapor, so permitting a thin intensifying screen to be produced, without using a carrier of glass.

Instead of forming an antimony-cesium photo-electric layer, a tri-alkali photo-electric layer may be used in which the antimony is activated in turn with each of its alkali metals, sodium, potassium and cesium.

In order that the invention may be more fully understood, one embodiment thereof will be more fully described by way of example, with reference to the accompanying drawings of which:

FIGURE 1 is a diagram representing a section of part of an electron image intensifying screen, the thickness thereof being greatly exaggerated for clearness, and

FIGURE 2 is a diagram showing parts of an electron optical device comprising an electron image intensifying screen of the form shown in FIGURE 1.

In FIGURE 1, an electron image intensifying screen 10 comprises a light-transmitting layer 1 of zinc sulphide activated with manganese mounted on a carrier layer 30; On the front face of the layer 1 is provided a photoelectric layer 2 of antimony activated with cesium. On the rear face of the carrier layer 30 is provided an electron permeable layer 3 of aluminum.

One method of manufacturing the electron image intensifying screen 10 consists in first forming as a carrier a layer 30 of aluminum oxide. This layer 30 may be suported on a wire frame and it then serves as a support on which the zinc sulphide layer 1 is deposited. The zinc sulphide may be deposited by mounting the carrier layer 30 in a vacuum chamber and evaporating zinc sulphide to form a crystalline layer on the layer 30. The known technique whereby zinc sulphide is evaporated from a molybdenum boat may be ,used, and the layer formed is of thickness between 1,11. and 10 The zinc sulphide layer is then activated by baking. The aluminum oxide layer 30 is electron-permeable so that the aluminum layer 3 may be deposited on top of the aluminum oxide layer 30. Alternatively, the aluminum oxide layer 30 may be mounted on a frame of hard glass or silica, again enabling it to be baked to a high temperature. It may conveniently be about 1000 A. (i.e. 0.1a) thick. Such carrier layers are very transparent and robust. After evaporation of the zinc sulphide phosphor layer on to one surface of such an aluminum oxide film the phosphor layer is reactivated by baking to a high temperature of about 1000 C.

Either before or after mounting aluminum oxide/phosphor screen (layers 30 and 1) in the tube in which it is to be used a layer 2 of antimony is evaporated on to the phosphor surface to a thickness less than the thickness of the fluorescent layer 1, and a layer 3 of aluminum is evaporated on to the aluminum oxide surface, the antimony layer 2 being subsequently activated in the well known manner with cesium vapor to form the photoelectric compound SbCs It is of note that the zinc sulphide phosphor would be attacked and disintegrated by exposure to cesium vapor, and the fluorescent elficiency of the zinc sulphide would be destroyed. However, in the method described above, the zinc sulphide layer 1 is covered on one side by the layer of aluminum and on the other side by the layer of antimony. The cesium vapor will not permeate either of these covering layers to attack the Zinc sulphide.

The operation of the electron image intensifying screen Will now be explained by reference to FIGURE 1. In the electron optical device in which it is to be used, the screen is mounted with the fluorescent layer 1 at the focus of an electron image. The aluminum layer 3 faces in the direction of incident high-energy electrons forming the said electron image. The path of one such incident high-energy electron is shown by the chain line 4. The electron passes through the electron-permeable aluminum layer 3 and the electron-permeable carrier layer 30 of aluminum oxide, and at some point 5 within the fluorescent layer 1, liberates photons which radiate from the point 5. Possible paths of such photons are shown in FIGURE 1 by the dotted lines 6 and 8. It will be seen that the lines 6 lead directly to the photo-electric layer 2 and the incident photons liberate electrons in the layer 2. By means of a suitable electric field, these electrons are drawn away from the surface of the photoelectric layer 2, the paths of the electrons being shown by the chain lines 9.

Other photons liberated at the point 5 travel away from the layer 2. These photons are reflected at the surface 7 of the aluminum layer 3 so that they follow paths such as those shown by the dotted lines 8. These photons will therefore fall on the photo-electric layer 2 after reflection and will liberate electrons therein in similar manner to the photons arrying by paths 6. These electrons similarly leave the photo-cathode 2 by paths 9.

Photo-electrons will be liberated from the surface of the photo-electric layer 2 over an area the diameter of which is approximately double the thickness of the zinc sulphide layer. In order that the definition of the electron image incident at 4 is not excessively impaired, it is desirable for the zinc sulphide layer 1 to be made as thin as possible. In FIGURE 2, an electron optical tube comprises an intensifying screen 10 of the form described with reference to FIGURE 1. In FIGURE 2, the layers 1, 2, 3 and of the screen 10 correspond to the fluorescent layer, the photo-electric layer, the aluminum layer, and the carrier layer, respectively, as shown in FIGURE 1. The tube 20 comprises an evacuated envelope 21 having transparent end-faces 22. In addition to the screen 10, the tube 20 has a photo-cathode 11 arranged at the front of the tube and a fluorescent screen 17 arranged at the opposite end thereof. Other electrodes of the tube 20. are not shown. An axial magnetic field between the photo-cathode 11 and the screen 10 is maintained by a solenoid 12. The screen'lt) is maintained positive with respect to the photo-cathode 11 by a potential source 13. Similarly, the solenoid 18 provides an axial magnetic field between the screen 10 and the fluorescent screen 17. The screen 17 is maintained positive with respect to the screen 10 by a potential source 19.

An object 14 provides divergent light rays 15 which are concentrated by a lens system 16 to form an image which is focussed on the surface of the photo-electric cathode 11. Between the photo-cathode 11 and the screen 10, the tube 20 comprises a single stage image intensifier the electron image formed being focussed on the screen 10. The incident electrons forming this electron image liberate photons in the fluorescent layer 1 and these in turn liberate further electrons in the photo-elec tric layer 2 in the manner already explained with reference to FIGURE 1. The electrons leaving the photo-electric layer 2 are greater in number than the incident electrons and are accelerated and focussed on to the fluorescent screen 17, so that an optical image is formed on the screen 17 which is an intensified reproduction of the image formed by the lens system 16 on the surface of the photoeathode 11.

It will be appreciated that the application of such an electron image intensifying screen is not limited to an optical image intensifying device and such a screen may be used, for example, to intensify the electron image formed in a television camera tube.

I claim:

1. A multiple layer electron image intensifying screen comprising a light-transmitting layer of fluorescent material in the amorphous state having a thickness between 1 and 10,, an electron permeable supporting carrier layer adjacent one side of the fluorescent material layer and having a thickness of about 0.1;t, a photo-electric layer adjacent the other side of the fluorescent material layer, and an electron permeable metallic layer adjacent the carrier layer, the total thickness of the screen being not more than 20,11.

2. A multiple layer electron image intensifying screen as claimed in claim 1, in which the carrier layer is a film of aluminum oxide.

3. A multiple layer electron image intensifying screen as claimed in claim 1, in which the carrier layer is a film of magnesium oxide.

4. A multiple layer electron image intensifying screen as claimed in claim 1, in which the thickness of the photoelectric layer is less than the thickness of the fluorescent layer.

References Cited by the Examiner UNITED STATES PATENTS 2,527,981 10/1950 Bramley 117210 X 2,606,299 8/1952 Coltman et al 313 2,700,116 1/1955 Sheldon 31365 3,054,917 9/ 1962 Eberhardt 117-210 X 3,148,297 9/1964 Schneeberger et al. 31365 OTHER REFERENCES Feldman and OHara, Formation of Films by Evaporation, Journal of the Optical Society of America, vol. 47, No. 4 (note pages 300-305).

WILLIAM D. MARTIN, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

R. E. ZIMMERMAN, S. W. ROTHSTEIN,

Assistant Examiners. 

1. A MULTIPLE LAYER ELECTRON IMAGE INTENSIFYING SCREEN COMPRISING A LIGHT-TRANSMITTING LAYER OF FLUORESCENT MATERIAL IN THE AMORPHOUS STATE HAVING A THICKNESS BETWEEN 1U AND 10U, AND ELECTRON PERMEABLE SUPPORTING CARRIER LAYER ADJACENT ONE SSIDE OF THE FLUORESCENT MATERIAL LAYER AND HAVING A THICKNESS OF ABOUT 0.1U, A PHOTO-ELECTRIC LAYER ADJACENT THE OTHER SIDE OF THE FLUORESCENT MATERIAL LAYER, AND AN ELECTRON PERMEABLE METALLIC LAYER ADJACENT THE CARRIER LAYER, THE TOTAL THICKNESS OF THE SCREEN BEING NOT MORE THAN 20U. 